The present invention relates to a extrusion dies for the extrusion of honeycomb structures from plasticized organic or inorganic batch materials. In particular, the present invention relates to extrusion dies with varying pin size.
Conventional extrusion dies for honeycomb extrusion comprise a feed or inlet section, provided with a plurality of feedholes for the input of extrudable material to the die, and a discharge section connecting with the feed section for reforming and discharging the extrudable material form a discharge face of the die. As discharged, the material is reformed into a honeycomb shape comprising a plurality of open-ended channels, equal in cross-section and bounded by interconnecting cell wall extending from one end of the structure to another in the direction of extrusion.
The discharge opening in the discharge face of these dies may be configured to form any of a variety of shapes for the interconnecting honeycomb wall structure. Currently, the discharge openings used for the extrusion of commercial ceramic honeycombs for treating automotive and diesel exhaust gases are formed by a criss-crossing array of long straight discharge slots of equal spacing. These long slots intersect to form a network of shorter slot segments for the forming of straight wall for square- or triangular-celled honeycombs.
Extrudable material processed through these commercial dies must follow a complex flow path. The material first moves from each feedhole through a transition zone into the base of the slot array, where it flows laterally to join with material from adjacent feedholes. Thereafter, the knitted material is again directed forwardly in the direction of feedhole flow toward the discharge opening formed by the slots, being discharged therefrom in the form of an array of interconnecting xe2x80x9cwebsxe2x80x9d or wall portions forming the channel walls of the honeycomb.
The cross-sectional shapes of the islands or xe2x80x9cpinsxe2x80x9d formed on the discharge faces of these extrusion dies by the intersecting slots segments govern the internal shapes of the channels in the extrudate. Presently, with conventional slotting these pins are equal in cross-section.
Recently, honeycomb structures with alternating channel diameters have generated interest for diesel filtration applications due to lower pressure drops across the length of the structure and increased resistance to damage during the required filter regeneration cycling. Co-pending application entitled xe2x80x9cHoneycomb with Alternating Cell Channel Diametersxe2x80x9d by D. M. Beall and J. Marcher, co-assigned to the present assigned, discloses such structures.
Current commercial methods are not adapted to the production of dies of varying pin sizes. Although such dies could be provided by any one of a number of known methods, including the assembly of arrays of plates as disclosed in U.S. Pat. No. 4,468,365 or by bonding pin arrays to a suitable die base plate as described in U.S. Pat. No. 5,761,787, these methods are not as effective or economic as current commercial methods of drilling a feedhole array and then cutting a discharge slot array on opposing surfaces of a metal die blank. There is, accordingly, a clear need for, an thus an object of the present invention is to provide for, a method of fabrication of dies with pin arrays comprising pins of alternating size.
Accordingly, the object of the present invention is directed at providing a die fabrication method wherein a honeycomb extrusion die incorporating a discharge face pin array of invariant square or rectangular pin size and shape is modified by electrical discharge machining (EDM) to convert the array to one of alternating pin size. The EDM method employed, referred to as plunge EDM, involves removing material from the sides of alternating pins in the array using an electrical discharge electrode of tabbed shape.
Specifically, the invention relates to a method of making a honeycomb extrusion die by a) providing a die body incorporating an inlet face, a discharge face opposite the inlet face, a plurality of feedholes extending from the inlet face into the body, and an intersecting array of discharge slots extending into the body from the discharge face to connect with the feed holes at feed hole intersections within the die, the intersecting array of discharge slots being formed by side surfaces of an array of pins; b) providing an electrical discharge electrode of equal size to the die body and comprising multiple rows of parallel-aligned tabs; c) bringing the electrical discharge electrode into contact with the array of pins on the discharge face of the die body; and, d) reducing alternate pins symmetrically in size by plunge electrical discharge machining to form an array of pins varying in size on the discharge face of the die body.